There are many requirements as well as metering devices for maintaining and changing fluid flow rates. For example, in certain applications it is necessary to turn on or shut off a preset fluid flow over a short period of time. Another type of application is where a flow rate must be selected with precision, for example, for carrier gas in atomic absorption spectrometric instruments, or in gas or liquid chromatography. In such instruments with electronic and computerized controls, the older conventional methods such as motors connected to conventional flow meters for setting and changing flows have become cumbersome and not readily adaptable, particularly where precision is required.
Most conventional flow meters are adjustable continuously over the operational range of flow rates and may not have the required precision. Another class of flow controllers utilizes an arrangement of binary flow control devices. These place a plurality of fluid lines in parallel, each having an on-off valve in series with a fixed flow control such as a precision orifice. The orifice is a different size in each line, or in at least some of the lines, so that combinations selected with the valves select fluid flow rate in increments. In one type of system, each fixed flow line passes fluid at a rate that is a factor of two greater than a preceding line in a sequence of the lines, as disclosed in U.S. Pat. Nos. 2,999,482, 3,726,296, 3,746,041 and 4,768,544. Other relative orifice sizes may be combined, for example as disclosed in U.S. Pat. No. 4,030,523 to attain steps in increments of 10% of the maximum flow, and U.S. Pat. No. 4,628,961 which discloses a stepper valve with equal flows.
A more recent class of devices for controlling fluid flow rates are formed in miniature solid state semiconductor chips and other materials using techniques similar to those of forming integrated electronic circuits. Such devices and techniques are described in detail in U.S. Pat. No. 4,821,997. A specific device is a Fluistor.TM. as described in an article "The Fluistor Microstructure is Poised for Commercialization" (no author), Sensor Business Digest, 1, No. 5, 9-11 (February 1992).